The present invention relates to colour display apparatus such as television displays, colour computer displays and colour printers, and, in particular, the display of colour images on a raster colour display apparatus.
Colours are often displayed on a computer display according to a particular model. The red, green, blue (RGB) colour model is one that is in common use with Cathode Ray Tubes (CRT) and colour raster display devices. Other colour display models include cyan, magenta, yellow (CMY) often used in colour-printing devices. An example of the use of the RGB model is in the NTSC picture display standard which is commonly used with computer displays. In this standard, each pixel element of the screen is divided into three separate sub groupings. These separate subgroupings or pixel elements are used to show the portions of Red, Green and Blue of a given pixel colour respectively.
The prior art shall now be described with reference to the following drawings in which:
FIG. 1 is a schematic view of a pixel of a CRT type device;
FIG. 2 is a schematic view of a pixel having Red, Green, Blue and White pixel elements; and
FIG. 3 is a schematic view of the colour gamut of the CRT type device as shown in FIG. 1.
As shown in FIG. 1, the viewing surface of a colour CRT consists of a large number of closely spaced pixels 1. Each pixel 1 is made up of a red (R), green(G) and blue (B) phosphor dot or pixel element. These dots are generally sized such that light emanating from the individual dots is perceived by the viewer as a mixture of the corresponding three colours. A wide range of different colours can thus be produced from a pixel by pixel variation of the strength with which each phosphor dot is excited. A conversion arrangement (not shown) is normally provided so that the excitation of each phosphor dot has some proportionality to the value of each of the above mentioned pixel element subgrouping. By way of example, a 24 bit per pixel colour display system divided into 8 bits for each of the three colours red, green and blue is often used. This corresponds to 28 or 256 separate intensity levels of each of red, green and blue respectively giving a total of 224 different colour values. A colour display capable of displaying this many colours can approximate a continuous tone image to such a degree that for all practical purposes the display can be considered to be a continuous tone display.
In order to conceptualize the range of colours that can be displayed by this method it is helpful to map these colours into a unit cube 3 as shown in FIG. 3. The individual contributions of each of the three separate subgroups at any one particular point are added together to yield the final colour. The directions of increased intensity of a given primary colour is shown by the axis 4 with green increasing to the right, blue increasing vertically, and red increasing into the plane of the page. The chroma or saturation of a particular colour is represented by the shortest distance of a colour from the main diagonal of the cube. The main diagonal of the cube, running from (0,0,0) to (1,1,1), with equal amounts of each primary, represents the different grey levels or grey scale, from black (0,0,0) to white (1,1,1).
The actual colours that can be displayed by a device will then form points within and on the surface of the cube""s volume. For example, in the 24 bit per pixel display model, the 256 different intensity levels of displaying the colour blue will form 256 separate horizontal planes through the cube. Similarly for the red and green intensity levels. The intersection of these planes will form 224 separate points within, and on the surface of, the cube.
Many display devices are unable to actually display the full range of colours provided by, for example, a 24 bit input pixel. For example, a black and white raster image display can only display two colours, black and white. Other colour display devices can only display a finite number of discrete intensity levels for each pixel element.
Recently, methods have been developed to increase the number of colours displayable on an discrete colour display device. The methods used are known generally as halftoning. For an explanation of the different aspects of halftoning the reader is referred to the book xe2x80x98Digital Halftoningxe2x80x99 by Robert Ulichney, published in 1991 by MIT Press.
For an example of a colour halftoning method, reference is made to an error diffusion method disclosed in xe2x80x98Color Image Quantization for Frame Buffer Displayxe2x80x99 by Paul Heckbert, published in Computer Graphics, Volume 16, Number 3, July 1982, pages 297-304, herein after referred to as the Heckbert process.
In the Heckbert process, the different possible output values of the display are chosen as the representative colours of the colour gamut of displayable images. A vector measure in a 3-dimensional colour space representing the distance between the nearest displayable colour value and a current input colour value is computed and this value is then preferably added to neighbouring pixels using a Floyd and Steinberg process (known per se).
However, not all display devices display the same range of colours, hence consideration must be given to what colour should actually be displayed when an input pixel""s colour lies outside the range or gamut of the displayable colours of a particular device.
The use of the above mentioned RGB colour model is helpful when each pixel of the display device comprises red, green and blue elements. However, it can sometimes be the case that the display device being used is not a RGB type device. For example, FIG. 2 shows a representation of a display device where each pixel is made up of a Red, Green, Blue and White (RGBW) element.
It is an object of the present invention to provide a means that permits the presentation of information according to one data format and gamut on a display that displays data according to a different data format and gamut.
In accordance with one aspect of the present invention there is disclosed a method for displaying a colour image on a display device, said colour image being provided as input colours in an input colour format, said method comprising the steps of:
mapping those output colours able to be displayed by the display device into a 3-dimensional vector space;
forming a volume enclosing all the output colours of the display device;
mapping each input colour into the 3-dimensional vector space;
moving any of the input colours outside the volume to corresponding points inside or on a surface of the volume; and
halftoning the input colours to produce an image that can be displayed on the display device.
In accordance with another aspect of the invention there is provided an apparatus for displaying a colour image on a display device, said colour image being provided as input colours in an input colour format, said display device capable of displaying a subset of said input colours, said apparatus comprising a plurality of channel processing means, said channel processing means comprising:
undisplayable colour determination means adapted to determine those undisplayable colours outside the displayable gamut of the display device and
remapping means connected to said undisplayable colour determination means and adapted to alter said undisplayable colours to be displayable colours.